Hi Roberthornman,
I held back posting a reply hoping someone might know the metric equivalent. I do know that the inch size is .275-40 tpi.
Also would think that you could find a used 3 jaw chuck on ebay using this sized WW collet. My 8mm 3 jaws have both WW collets and Moseley long conoidal collets so I know they were available at one time.
Sorry couldn’t help more Robert,
Bob